there is no such thing as a do it all rod. If you want to fish effectively, you need the right tools for the job. A 9' 5 is good for throwing dries in most situations as well as nymphs of average size in a medium size river. Its not a very good streamer rod or very good at turning over hoppers in the wind (action of the rod has alot to do with that to). I fish in MT alot, especially on the Yellowstone and Missouri as well as some small creeks and medium rivers like the Bitteroot and Big Hole. I like a 9' 4 weight for dries and some light nymphing on less windy days, For streamers I fish a 9' 7 weight because I can throw heavier lines and bigger flies easier and farther. For hoppers or other big dries and nymphing all day I like a 9' 6 weight. I also bring a couple specialty rods like a 6/7 switch for swinging and a 5'9" 3 weight glass rod for small creeks and brookie ponds.
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Since you're asking for advice on rod selection, I'm going to assume you don't know what you need. Therefore I'll happily share my biases with you. You'll need at least two rods, not just one. Start with that 4 or 5 wt in a length that suits you. And as other posts note, it's windy at times, so you'll want a 6 wt, or maybe even a 7. And since trout rise better to bamboo, you should buy yourself a pair of split cane rods. There's a nice shop in Twin Bridges that makes some of the nicest cane rods anywhere. Fish bamboo for trout in Montana. You'll never regret it.

The rod I use most in MT is a medium-fast 10' 4wt. It gets my line above the tall grass on the bank, roll casts, and mends really well. It is more versatile than my medium-fast 9' 5wt, gets the same distance with less effort, and nymphs better.

I agree: an 8 1/2' four weight and a 9' (or longer) six weight are the perfect pair for the common, middle range of trout fishing almost anywhere. The rods you'd use for Jurassic Lake or the East Fork of Mink Brook are specialized extremes.

Having used five weights many years in many places, I've come to think of the five weight as a useful compromise, but seldom a perfect fit for the situation of the hour.

Basically what you do is rig up two rods. One dor nymphs/streamers etc... take the rod you are going to use second and put it down the front of your waders to the wading belt or just past. I ptefer my left side, putting it towards my left leg.

When the time comes to switch just put the fly on the hook keeper, grab the other rod and switch. Takes at max 30 seconds or so! Also you have your other rod wih you and wont leave it on a random bank

An alternative to rigging and bringing two rods is to bring a relatively fast action 9' 5wt and fish dries and smaller waters with a DT5F line and windy days and bigger waters with a WF6F line. I think most fast rods these days can easily handle the higher line weights. If big streamers are your bag then also pack a Wulff 5 wt Ambush line, which will carry a pretty big fly with minimal false casting. I use a Scott 9' 5 wt STS when fishing Montana and carry the DT5 and WF6s on spools with me all the time. I am sure that with this strategy of various lines you can come up combinations for rods and lines that will best fit your situation and minimize the gear you have to carry on the stream. Good luck in Montana, you will have a great time even if you only have one rod and one line.

I see your point Red Arch, I wasn't assuming that he was changing back and forth all day to meet conditions. Usually when it gets windy it is windy for a while and taking the three or four minutes to change doesn't seem that long. Now if there is a fish rising then three minutes is an eternity! I do have one question, if it begins to rain and you have to put on your raincoat then how do you handle the second rod if you want to stay and fish? Does it stick up through the top of your raincoat? I have gotten rained on in Montana a number of times on an otherwise kind of sunny day. I have at times carried a second rod but never tried keeping it in the front of my waders, thus the extra spools instead of a second rod.